Gouty arthritis

Deposition of MonoSodium Urate (MSU)
> 40 years
Male > female

Tophus:
Hallmark of gout
Tophus = urate, protein matrix, inflammatory cells and gaint cells
Tophus seen in intra-articular (synovial) space, periarticular subcutaneous tissue, tendon, ligament, cartilage, bone, bursa
Common areas: olecranon bursa, ear, nose, meniscus, quadriceps tendon, patellar tendon, Achillis tendon (may lead to tendon rupture)
Subcutaneous tophi may ulcerate or may produce saucerization of the bone

Key points:
Most common joint: 1st MTPJ
Next common sites:  IPJ of the big toe, TMTJs
Typically monoarticular
When polyarticular: asymmetric
Pedilication for extremity joints, lower limb more common than upper limb
Axial joints rarely affected
Can lead to carpal tunnel syndrome, trigger finger, discitis, paraplegia

Plain film:
5-10 year delay in first radiographic presentation!
Earliest: soft tissue swelling
Then: fine lacy periosteal reaction (this is actually urate crystals adjacent to the cortex)
Faint calcification in the soft tissue (tophi), seen in 50%
Intracortical erosion and irregularity
Osteochondral compression/ cupping
JUXTRA ARTICULAR PUNCHED OUT EROSIONS WITH SCLEROTIC MARGINS AND OVERHANGING MARGING (seen late, but characteristic)
'Rat bite' from adjacent tophi
Mushrooming/ bulbous appearance of the bone ends

JOINT SPACE IS PRESERVED TILL LATE
NO periarticular osteopenia (diffuse osteopenis - late in disease)
Introsseous tophi - seen as cysts (rarely sclerotic lesion)
Very late: pencil-in-cup, subluxation, bone infarct, arthritis mutilans, ankylosis

USG:
Three times sensitive to detect erosions of smaller than 2 mm, compared to plain radiograph
'Double counter sign' - hypperechoic, irregular band over the articular cartilage (secondary to MSU deposit)
Hyperechoic soft tissue +/- posterior shadowing
'Snow-storm synovial effusion'
Synovial hypertrophy
Increased vascularity
Tophi - hypo-to-hyperechoic, heterogenous, thin anechoic rim

CT:
Tophus as  hyperdense mass (>160 HU)
Dual energy CT (80 and 140kV) can assess the chemical composition
Superior to MR in detection of erosions!

MRI:
Early detection of tophi
Synovial thickening
Joint effusion
Bone erosion
Bone marrow edema
Tophi: homogenous, low-to-intermediate on T1 and T2, and nehance intensely

Reference:
1. S. Dhanda, A re-look at an old disease: A multimodality review on gout, Clinical Radiology, Volume 66, Issue 10, October 2011, Pages 984-992
2. Learning radiology